This invention relates to AC drives and, in particular, to a device and method for regulating the current supplied by an inverter to an induction motor in response to switching signals from a waveform generator.
Given that motor curlrents must be accurately controlled to eliminate the risk of inverter trips, it is necessary to eliminate or reduce the disadvantages of current control. These disadvantages include the need for more voltage headroom on the inverter bus, less than optimum inverter switching signals, poorly defined switching frequency, and significant errors in current regulation at high stator frequencies.
There are several basic approaches to current control. Control of current in the stationary reference frame using bang-bang control or triangular modulated control is the most common. The characteristics of these regulators have been studied and they are known to require a great deal of voltage headroom and to result in large current errors under some circumstances.
The next level of control is to regulate the currents in either a stationary or synchronous reference frame by taking into account that only seven discrete voltage vectors can be applied to a motor. At each sample interval, a new voltage vector is chosen based on the present current error and an estimate of back EMF. In general, these schemes require some sort of comparator which chooses different voltage vectors under rapidly changing dynamic conditions than the voltage vectors which would be chosen to correct only small current errors. In this way, the available inverter switching frequency is better utilized. However, these schemes are subject to loss of current control if orientation is lost.
For optimum current control, a current regulator is provided to generate voltage magnitude and phase information which is supplied to a conventional voltage mode PWM waveform generator which chooses multiple voltage vectors to generate the required average voltage. This technique provides optimum inverter gating signals. When combined with a digital control system so that the regulated current reaches its set point in one sample time, the current regulator provides excellent dynamic current control without the need for a large voltage headroom. Limiting of the demanded motor voltage based on the known constraints of the available inverter bus voltage and the PWM waveform modulation index provides a further decrease in the necessary headroom.